The present invention relates to tire molds and, more specifically, to techniques for venting such molds.
It is known that it is necessary to eliminate the air which remains imprisoned between the molding surface of the molds and the raw rubber of the tire. In order to assure the complete venting of the mold, current practice is to provide small holes approximately everywhere on the molding surface where there is the danger of gas remaining imprisoned without being able to escape between the parts of the mold, for instance without being able to escape between shell and sectors, or between the sectors themselves.
These venting devices have two types of disadvantages. On the one hand, they leave an undesirable marking on the surface of the tire after its vulcanization and removal from the mold; the flow of the rubber into the interior of the holes causes the creation of small roughnesses or even larger tubular protuberances. Furthermore, these vent holes can easily become clogged, in particular because the rubber which has flowed therein may remain caught in the hole after vulcanization. Cleaning of the mold is then indispensable, and it is complicated by the fact that it impossible to dissolve the clogging materials chemically.
For this reason, the state of the art also knows a large number of more elaborate proposals, all directed at solving this problem. Mention may be made of very complicated vents (U.S. Pat. Nos. 4,347,212 or 4,426,497), in a mold made by stacking plates in order to obtain natural venting between the plates (U.S. Pat. No. 4,691,431), or else the use of porous materials for the production of the mold (GB Patent 843 300) or a part thereof (WO 88/01927).
However, despite the extensive research which has been carried out on this subject, the problem still remains that all the solutions proposed up to the present time either hamper the venting too much in order for it to be effective or cannot be used in the tread, at least with certain types of tread patterns, such as those with many sipes cleats, or else they are too expensive or too complicated to use on an industrial scale (aspiration by partial vacuum).